Modulation of RNA expression by intracellular calcium. Existence of a threshold calcium concentration for induction of VL30 RNA by epidermal growth factor, endothelin, and protein kinase C

We investigated the role of intracellular [Ca²⁺] in mediating the independent signal transduction pathways leading to induction of VL30 RNA expression by multiple agonists in the Rat-1-derived RVL-3 cell line. This cell line contains a single integrated VL30 element, nd displays a rapid transcriptional activation of VL30 following stimulation by epidermal growth factor, endothelin, or the phorbol ester tumor promoter 12-O-tetradecanoylphorbol acetate (Roland, K.D., Brown, A.M.C., and Magun, B.E. (1987) M. Cell. Biol. 7, 2296-2298). Neither epidermal growth factor nor endothelin is dependent upon protein kinase C for activation of VL30 expression, as both of these agonists induce normal levels of VL30 RNA expression, even in cells which have been severely depleted of protein kinase C following chronic 12-O-tetradecanoylphorbol acetate exposure (2). Induction of VL30 RNA expression by either endothelin or 12-O-tetradecanoylphorbol acetate was blocked by concomitant exposure of RVL-3 cells to the intracellular Ca²⁺-chelating agent 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid at a concentration sufficient to buffer intracellular [Ca²⁺] below 200 nM, and VL30 RNA was induced by the application of the Ca²⁺ ionophore A23187 in the absence of agonist. Normal levels of VL30 expression in response to epidermal growth factor were observed at 165 nM [Ca²⁺], but were significantly inhibited at 115 nM [Ca²⁺]. Both the protein kinase C-dependent and protein kinase C-independent pathways leading to VL30 transcription were dependent upon the presence of an intracellular [Ca²⁺] exceeding 115 nM. The dependence upon intracellular Ca²⁺ transients for transcriptional induction by endothelin appears to be a characteristic of VL30 expression, as 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid treatment did not prevent the endothelin-induced transcription of the protooncogenes c-jun and c-fos.